Technical Field
The disclosure in generally relates to a memory device, and more particularly to a three dimensional (3D) memory device.
Description of the Related Art
Non-volatile memory (NVM) which is able to continually store information even when the supply of electricity is removed from the device containing the NVM cell has been widely adopted by bulk solid state memory applications in portable audiovisual entertainment devices, cell phones or digital cameras etc. Recently, various three dimensional (3D) memory devices, such as a 3D flash memory device having a single gate, a double gate or a surrounding gate, has been provided in order to accommodate the rising demand for superior memory.
A typical 3D memory device includes a 3D memory cell array having vertical channels formed in a multi-layer stacks. To take a single-gate vertical channel (SGVC) NAND device for example, a plurality of stacked poly-silicon conductive stripes are applied to serve as the gates of the memory cell involved in the 3D memory cell array. The poly-silicon conductive stripes should be divided to several portions for connecting to a plurality of metal word lines disposed on the cell memory array through a plurality of staircase-shaped word lines contacts.
However, the word lines contacts occupy the areas of the SGVC NAND device, and the space over the memory cell array for arranging the word lines is rather limited to allow additional word lines passing there through. As the memory density of the SGVC NAND device is increased, the levels of the conductive strips can also be increased, and more word lines contacts and word lines are required for the added levels of the conductive strips. Currently, there are two ways to solve these problems. One is to increase the physical size of the SGVC NAND device, the other is to shrink the pitch of the word line for allowing the added word lines contacts and word lines arranged therein.
But, the process window and the yield for fabrication the SGVC NAND device may be decreased, and oxide breakdown may occur due to the shrunk pitch of the word lines. The production cost thereof may be thus increased. In contrast, increasing the physical size of the SGVC NAND device does not meet the current trend of being lightweight and compact in memory device.
Therefore, there is a need of providing an improved 3D memory device to obviate the drawbacks encountered from the prior art.